(1) Field of the Invention
The invention relates to a device for determining the dew point temperature of a measurement gas, having a light guide, a condensation surface located on the light guide and whose reflectivity is dependent on the condensation of the measurement gas, a light source for emitting light through the light guide onto the condensation surface, a light sensor for determining the light intensity reflected back into the light guide by the condensation surface and means for adjusting the temperature of the condensation surface, according to the preamble of claim 1.
(2) Description of Related Art Including Information Disclosed under 37 CFR 1.97 and 1.98
Dew point sensors and methods for determining the dew point temperature of a measurement gas are known. These sensors and methods are based on the principle that as a function of its temperature a gas is in a position to solely absorb a certain water vapour quantity, which rises with increasing temperature.
If a water vapour-containing gas is cooled to below the so-called dew point temperature, excess water vapour is precipitated and condensation occurs. This condensation effect can be utilized metrologically.
Thus, e.g. DE 199 15 095 A1 discloses a so-called dew point mirror hygrometer, which has a dew point mirror on which shines a light source and which is in contact with the measurement gas. The light reflected and/or scattered by the dew point mirror is detected by a photoreceiver. If as a result of a change in the dew point mirror temperature the measurement gas dew point temperature is reached, condensate is deposited on said mirror. At this time there is a change to the reflectivity and/or diffusing power of the dew point mirror and consequently the light intensity detected by the photoreceiver also changes.
In the case of the device known from DE 199 15 095 A1 the light is passed from the light source to the dew point mirror and from there to the photoreceiver directly through the measurement gas. In this case, the measurement signal at the photoreceiver can be falsified by any impurities, water droplets and/or ice crystals present in the measurement gas and which also absorb and/or scatter light. In addition, any impurities present in the measurement gas can be deposited on the dew point mirror surface, which can also falsify the measurement signal.
Another device for determining the dew point temperature is described in DE 200 12 060 U1. In said device the light path runs from the light source to the light sensor essentially through a medium differing from the measurement gas. This largely prevents an undesired absorption and/or scattering of the light through the measurement gas and also an undesired contamination of the light path. According to the teaching of DE 200 12 060 U1 the light is passed through a light guide to condensation areas located on the light guide surface and which are in contact with the measurement gas. If a condensate is deposited in the condensation areas, there is a change there to the critical angle for the total reflection of the light. This can lead to a coupling or feeding out of light and this can be detected at the light sensor as a change to the light intensity. To concentrate the condensation of the measurement gas on the condensation areas, according to DE 200 12 060 U1 said areas have a hydrophilic surface, whereas the remaining areas are hydrophobic.
U.S. Pat. No. 3,528,278 discloses another dew point sensor in which the light is passed through a light guide and on its surface is reflected in condensation-dependent manner back into the light guide.
DE 100 56 771 C2 proposes the provision on the sensitive surface of dew point humidity sensors of periodically arranged, hydrophilic surfaces, which are surrounded by hydrophobic areas. The hydrophilic surfaces can in particular be used to bring about a start of the condensation process before the dew point temperature is reached. The hydrophilic areas are wetted and with increasing condensation time droplets with a relatively steep wetting angle can be formed. In the case of sensors designed in this way, in certain circumstances comparatively long condensation times can arise and the sensors then react comparatively slowly.
DE 35 43 155 C2 discloses an optical dew point sensor with an optical waveguide having a roughening on its surface. If this roughening is unwetted, a considerable proportion of the light coupled or fed into the optical waveguide passes out through the roughening. However, if on dropping below the dew point the roughened area is wetted, the light intensity loss there is lower.
Another optical dew point hygrometer is known from EP 0 843 174 A1. This known dew point hydrometer detects a condensate coating which forms on a cooled, curved optical fibre.
U.S. Pat. No. 5,396,325 discloses another optical sensor permitting the detection of the formation of water droplets on a measurement surface on the basis of a change to the surface reflectivity.
The article by Lance D. Eske, David W. Galipeau “Characterization of SiO.sub.2 surface treatments using AFM, contact angles and a novel dew point technique”, published in Colloids and Surfaces A, 154 (1999), pp 33-51 describes the use of surface acoustic wave (SAW) sensors for detecting the bedewing of a surface.
Another dew point mirror hygrometer is described in the article “The measurement of tiny dew droplets at the initial deposition stage and dew point using a phase-shift interference microscope” by Shigeaki Matsumoto, published in the journal “Measurement Science and Technology”, 14 (2003), pp 2075-2080.